GENEVA — The worst Ebola outbreak on record is a public health emergency that threatens nations outside the four in West Africa where the virus is spreading, the World Health Organization said Friday.

The Geneva-based United Nations health agency stopped short of recommending a general travel and trade ban, saying only that infected people shouldn’t cross borders except as part of an official medical evacuation. Affected countries should work with airlines to ensure proper care for crews based there and enable quick identification of passengers who may have had contact with infected people, the WHO said.

“The possible consequences of further international spread are particularly serious in view of the virulence of the virus,” the WHO said in a statement. “A coordinated international response is deemed essential to stop and reverse the international spread of Ebola.”

The outbreak has killed 961 people in Guinea, Liberia, Nigeria and Sierra Leone since it was first reported in March, and 68 new cases were reported over two days, the WHO said Friday. It’s the first time Ebola has appeared in West Africa. A lack of border controls has allowed infected people who didn’t seek medical attention because of fear, suspicion or stigma to travel freely between the three countries. Unsanitary funeral practices involving contact with corpses have also fanned the spread of the disease.

Greater awareness of the disease is likely to lead to more patients coming forward to get treated, leading to higher numbers of reported cases, Director-General Margaret Chan said. Some people have claimed the disease doesn’t exist.

The WHO is “fully prepared to be responding to this outbreak at a high level for some number of months,” Assistant Director-General Keiji Fukuda said Friday in Geneva. “The likelihood is that it will get worse before it gets better.”

Declaring the epidemic a serious emergency should galvanize the response effort, Fukuda said. The WHO and aid organizations including Doctors Without Borders are “extremely stretched” by humanitarian crises around the globe as well as outbreaks of Middle East respiratory syndrome coronavirus and a new influenza virus, Chan said. She called on the international community to step up aid as soon as possible.

While the U.S. Centers for Disease Control and Prevention is contributing to the fight against the outbreak, a top official at a nonprofit treating patients in Liberia later said the U.S. and Europe essentially ignored the rising case numbers until two Americans, Kent Brantly and Nancy Writebol, became infected in July.

“That the world would allow two relief agencies to shoulder this burden along with the overwhelmed ministries of health in these countries testifies to the lack of serious attention the epidemic was given,” Ken Isaacs, vice president of international programs and government relations for Samaritan’s Purse, said in written testimony to Congress.

Doctors Without Borders, which has almost 700 workers in the three worst affected countries, said it can’t do more than it’s already doing.

“Lives are being lost because the response is too slow,” the group said. “Countries possessing necessary capacities must immediately dispatch available infectious disease experts and disaster relief assets to the region.”

Health experts from Uganda, Gabon and Congo who have dealt with past outbreaks are now in the region, adding to the care effort led by Doctors Without Borders and the Red Cross, Chan said. The European Union said Friday it will boost its Ebola assistance again this year, bringing the total to $16 million. The CDC has increased staffing in the affected areas, and two development banks are preparing as much as $260 million in funding.

Two Spanish nuns and a priest have also caught Ebola, Jose Maria Viadero, director of Spanish church-based aid agency Juan Ciudad, said in Madrid Friday. The priest is expected to recover, and the patients don’t want updates made public, he said.

“This is a wake-up call for the world community,” Viadero said. “The problem with treatment is that when people fall ill they don’t want to go to hospital; they hide because it’s such a stigmatizing disease.”

Witnesses from Samaritan’s Purse and SIM USA, another charity working in the region, described nightmarish scenes in Liberia, where doctors and nurses have fled the country or refuse to work, hospitals aren’t functioning and bodies of Ebola victims lie in the streets of the capital, Monrovia, or in shallow, unsanitary mass graves.

Protective equipment and proper training of medical workers, many of whom are volunteers, is one of the most urgent concerns, Fukuda said. Health-care workers who have become infected didn’t take adequate protective measures, he said.

Medical workers who are afraid of getting infected also need incentives to bring them back to work, Chan said.

“Governments in the affected countries need to send a strong signal that local health workers’ contributions are appreciated and they must be properly paid,” she said.

The Ebola virus is spread through direct contact with bodily fluids from an infected person. There is no approved cure. Standard treatment is to keep patients hydrated, replace lost blood and use antibiotics to fight off opportunistic infections. The hope is that the body’s immune system will eventually beat the disease.

While Ebola poses a serious threat for people in West Africa, there is a low probability that it will turn into a global pandemic, said Jeremy Farrar, director of the Wellcome Trust, one of the world’s biggest medical-research charities.

“Ebola is still of global concern and there remains a possibility that imported cases of Ebola will reach countries outside of West Africa and also that the health-care system, economies and stability of the countries involved will be affected,” Farrar said Friday in an emailed statement.

The WHO is also convening a panel of medical ethicists next week to explore the use of experimental treatments for Ebola after an experimental antibody cocktail developed by Mapp Biopharmaceutical Inc. was used to treat two infected American health workers.

The panel will weigh the use of the unapproved drugs in light of the supply constraints and limited evidence of their safety and efficacy in humans, Fukuda said. In the case of the two Americans treated with the drug, it’s unclear whether the treatment helped them recover or their own immune systems fought off the virus with supportive medical care, he said. More than 40 percent of people infected by the virus in this current outbreak have survived without drug treatment.

With assistance from Katie Linsell and Ben Sills in Madrid and Alex Wayne in Washington.

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